Internet Engineering Task Force (IETF)                     L. Velvindron
Request for Comments: 9155                                 cyberstorm.mu
Updates: 5246                                                K. Moriarty
Category: Standards Track                                            CIS
ISSN: 2070-1721                                               A. Ghedini
                                                         Cloudflare Inc.
                                                           December 2021

   Deprecating MD5 and SHA-1 Signature Hashes in TLS 1.2 and DTLS 1.2


   The MD5 and SHA-1 hashing algorithms are increasingly vulnerable to
   attack, and this document deprecates their use in TLS 1.2 and DTLS
   1.2 digital signatures.  However, this document does not deprecate
   SHA-1 with Hashed Message Authentication Code (HMAC), as used in
   record protection.  This document updates RFC 5246.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Signature Algorithms
   3.  Certificate Request
   4.  Server Key Exchange
   5.  Certificate Verify
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Authors' Addresses

1.  Introduction

   The usage of MD5 and SHA-1 for signature hashing in (D)TLS 1.2 is
   specified in [RFC5246].  MD5 and SHA-1 have been proven to be
   insecure, subject to collision attacks [Wang].  In 2011, [RFC6151]
   detailed the security considerations, including collision attacks for
   MD5.  NIST formally deprecated use of SHA-1 in 2011
   [NISTSP800-131A-R2] and disallowed its use for digital signatures at
   the end of 2013, based on both the attack described in [Wang] and the
   potential for brute-force attack.  In 2016, researchers from the
   National Institute for Research in Digital Science and Technology
   (INRIA) identified a new class of transcript collision attacks on TLS
   (and other protocols) that relies on efficient collision-finding
   algorithms on the underlying hash constructions
   [Transcript-Collision].  Further, in 2017, researchers from Google
   and Centrum Wiskunde & Informatica (CWI) Amsterdam [SHA-1-Collision]
   proved SHA-1 collision attacks were practical.  This document updates
   [RFC5246] in such a way that MD5 and SHA-1 MUST NOT be used for
   digital signatures.  However, this document does not deprecate SHA-1
   with HMAC, as used in record protection.  Note that the CA/Browser
   Forum (CABF) has also deprecated use of SHA-1 for use in certificate
   signatures [CABF].

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Signature Algorithms

   Clients MUST include the signature_algorithms extension.  Clients
   MUST NOT include MD5 and SHA-1 in this extension.

3.  Certificate Request

   Servers SHOULD NOT include MD5 and SHA-1 in CertificateRequest

4.  Server Key Exchange

   Servers MUST NOT include MD5 and SHA-1 in ServerKeyExchange messages.
   If the client receives a ServerKeyExchange message indicating MD5 or
   SHA-1, then it MUST abort the connection with an illegal_parameter

5.  Certificate Verify

   Clients MUST NOT include MD5 and SHA-1 in CertificateVerify messages.
   If a server receives a CertificateVerify message with MD5 or SHA-1,
   it MUST abort the connection with an illegal_parameter alert.

6.  IANA Considerations

   IANA has updated the "TLS SignatureScheme" registry by changing the
   recommended status of SHA-1-based signature schemes to "N" (not
   recommended), as defined by [RFC8447].  The following entries have
   been updated; other entries in the registry remain the same.

      | Value  |  Description   | Recommended |      Reference      |
      | 0x0201 | rsa_pkcs1_sha1 |      N      | [RFC8446] [RFC9155] |
      | 0x0203 |   ecdsa_sha1   |      N      | [RFC8446] [RFC9155] |

                                  Table 1

   IANA has also updated the reference for the "TLS SignatureAlgorithm"
   and "TLS HashAlgorithm" registries to refer to this document in
   addition to RFCs 5246 and 8447.

7.  Security Considerations

   Concerns with (D)TLS 1.2 implementations falling back to SHA-1 is an
   issue.  This document updates the TLS 1.2 specification [RFC5246] to
   deprecate support for MD5 and SHA-1 for digital signatures.  However,
   this document does not deprecate SHA-1 with HMAC, as used in record

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,

8.2.  Informative References

   [CABF]     CA/Browser Forum, "Ballot 118 -- SHA-1 Sunset (passed)",
              October 2014, <https://cabforum.org/2014/10/16/ballot-118-

              Barker, E. and A. Roginsky, "Transitioning the Use of
              Cryptographic Algorithms and Key Lengths", NIST Special
              Publication 800-131A, Revision 2,
              DOI 10.6028/NIST.SP.800-131Ar2, March 2019,

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,

              Stevens, M., Bursztein, E., Karpman, P., Albertini, A.,
              and Y. Markov, "The First Collision for Full SHA-1", 2017,

              Bhargavan, K. and G. Leurent, "Transcript Collision
              Attacks: Breaking Authentication in TLS, IKE, and SSH",
              DOI 10.14722/ndss.2016.23418, February 2016,

   [Wang]     Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
              Full SHA-1", DOI 10.1007/11535218_2, 2005,


   The authors would like to thank Hubert Kario for his help in writing
   the initial draft version of this document.  We are also grateful to
   Daniel Migault, Martin Thomson, Sean Turner, Christopher Wood, and
   David Cooper for their feedback.

Authors' Addresses

   Loganaden Velvindron
   Rose Hill

   Phone: +230 59762817
   Email: logan@cyberstorm.mu

   Kathleen Moriarty
   Center for Internet Security
   East Greenbush, NY
   United States of America

   Email: Kathleen.Moriarty.ietf@gmail.com

   Alessandro Ghedini
   Cloudflare Inc.

   Email: alessandro@cloudflare.com